1. The Field of the Invention
The invention generally relates to lasers. More specifically, the invention relates to Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Description of the Related Art
Lasers are commonly used in many modern components. One use that has recently become more common is the use of lasers in data networks. Lasers are used in many fiber optic communication systems to transmit digital data on a network. In one exemplary configuration, a laser may be modulated by digital data to produce an optical signal, including periods of light and dark output that represents a binary data stream. In actual practice, the lasers output a high optical output representing binary highs and a lower power optical output representing binary lows. To obtain quick reaction time, the laser is constantly on, but varies from a high optical output to a lower optical output.
Optical networks have various advantages over other types of networks such as copper wire based networks. For example, many existing copper wire networks operate at near maximum possible data transmission rates and at near maximum possible distances for copper wire technology. On the other hand, many existing optical networks exceed, both in data transmission rate and distance, the maximums that are possible for copper wire networks. That is, optical networks are able to reliably transmit data at higher rates over further distances than is possible with copper wire networks.
One type of laser that is used in optical data transmission is a Vertical Cavity Surface Emitting Laser (VCSEL). As its name implies, a VCSEL has a laser cavity that is sandwiched between and defined by two mirror stacks. A VCSEL is typically constructed on a semiconductor wafer such as Gallium Arsenide (GaAs). The VCSEL includes a bottom mirror constructed on the semiconductor wafer. Typically, the bottom mirror includes a number of mirror periods, each period including a high index of refraction layer and a low index of refraction layer. As light passes from a layer of one index of refraction to another, a portion of the light is reflected. By using a sufficient number of alternating layers, a high percentage of light can be reflected by the mirror. Typically, each layer within a mirror has a thickness of one-quarter wavelength of the optical field being produced in order to improve reflectivity.
An active region that includes a number of quantum wells is formed on the bottom mirror. The active region forms a PN junction sandwiched between the bottom mirror and a top mirror, which are of opposite conductivity type (i.e. a p-type mirror and an n-type mirror). Free carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected carriers form a population inversion in the quantum wells that produces optical gain. Optical gain occurs when photons in the active region cause electrons to transition from the conduction band to the valance band which produces additional photons. When the optical gain is equal to the loss in the two mirrors, laser oscillation occurs. The electrons in the quantum well conduction band are stimulated by photons to recombine with holes in the quantum well valence band. This process results in the stimulated emission of photons, which are coherent with the initiating photons.
The active region may also include an oxide aperture formed using one or more oxide layers formed in the top and/or bottom mirrors near the active region. The oxide aperture serves both to form an optical cavity and to direct the bias current through the central region of the cavity that is formed.
A top mirror is formed on the active region. The top mirror is similar to the bottom mirror in that it generally comprises a number of layers that alternate between a high index of refraction and a lower index of refraction. Generally, the top mirror has fewer mirror periods of alternating high index and low index of refraction layers, to enhance light emission from the top of the VCSEL.
Illustratively, the laser functions when a current is passed through the PN junction to inject free carriers into the active region. Recombination of the injected free carrier electrons from the conduction band quantum wells to the valence band quantum wells results in photons that begin to travel in the laser cavity defined by the mirrors. The mirrors reflect the photons back and forth. When the bias current is sufficient to produce a population inversion between the quantum well states at the wavelength supported by the cavity, optical gain is produced in the quantum wells. When the optical gain is equal to the cavity loss laser oscillation occurs and the laser is said to be at threshold bias and the VCSEL begins to ‘lase’ as the optically coherent photons are emitted from the top of the VCSEL.
In the design of a VCSEL, efforts are made to increase thermal conductivity, increase electrical conductivity, increase reflectivity, and minimize free carrier absorption. Typically, reflectivity is improved by designing the thickness of each of the mirror layers such that the nulls and peaks of the optical field occur within the compositional ramps between each of the mirror layers. Furthermore, electrical conduction is often improved by heavily doping areas within the VCSEL structure. However, elevated doping levels often comes at the cost of increasing absorption levels which reduces mirror reflectance and transmittance causing high threshold currents. Additionally, designing the thickness of the mirror layers such that the peaks and nulls of the optical field occur within the compositional ramps between the mirror layers can often reduce the conduction benefit that would otherwise be gained from elevated doping levels in the ramps because of the low mobilities in the ramps.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.